1. Field of the Invention
The present invention relates generally to optics and more particularly, to conformal fiber-optics vibrometry for real-time full-field measurement of the displacement and vibration of arbitrarily-shaped surfaces wherein the measurements are acquired by a conformal fiber-optics vibrometer array.
2. Description of the Related Art
A laser Doppler vibrometer (“LDV”) is essentially a laser interferometer designed specifically for remote, non-contact measurement of solid body dynamic displacement, velocity, and other physical parameters of interest using homodyne or heterodyne demodulation techniques. Laser interferometers include those broadly categorized as Michelson, Mach-Zehnder, Fabry-Perot, Sagnac, and Fizzeau. Interferometers commonly employ heterodyne detection techniques in which the frequency of the reference channel is offset from that of the signal channel. These interferometers can be bulk-optic or hybrid bulk, fiber-optics interferometers.
Recently, all-fiber and hybrid bulk, fiber-optics interferometers have been developed using, for example, a varied combination of fiber couplers, isolators, circulators, polarizers, phase and intensity modulators, fiber amplifiers, Bragg gratings, DFB laser diodes and wavelength-tunable fiber lasers. Thus, such fiber-optics technology is increasingly being employed with LDVs to provide a convenient and flexible non-contact method to measure displacement and vibration of a remote surface at a point on the structure illuminated by a single laser beam.
Currently, the global dynamics of an extended structure can only be determined by numerical (modal) analyses of data taken with a single point sensor, such as an accelerometer or LDV which thus has to be sequentially moved or scanned across the surface as the data are measured point by point. In traditional modal analyses, the frequency response function of the structure (FRF), which is the ratio of output response to input excitation, is determined at a number of separate locations; thus, additional means are required to measure the input force such as an attached accelerometer or secondary reference vibrometer. However, there are limitations with traditional modal analyses. For example, traditional modal analyses assume that (i) the input excitation can be accurately measured, which may be difficult or impossible in operational environments, (ii) the excitation and structural response is repeatable and largely insensitive to environmental factors for the duration of the measurement, and (iii) the structural dynamics are linear and time invariant (i.e. the principal of modal superposition is applicable).
Traditional modal analysis methods thus use a roving accelerometer in conjunction with fixed point impact (or vice versa), or swept sine excitation or using non-contact laser vibrometry, such as a single-beam LDV. However, there are limitations with such approaches. For example, with a single beam LDV, characterization of the motion of an extended area of the structure relies on the use of XY scanning galvanometer mirrors to reposition the laser beam over a grid of points on the surface. The inference of global structural dynamic behavior is further subject to additional restrictive assumptions concerning the actual structural behavior between sequentially scanned measurements.
In addition to the limitation of the restrictive assumptions implicit in traditional modal analyses, point measurements performed sequentially require application of repetitive stimulus for each point measurement, making this approach slow and unsuitable for the study of many system dynamics or diagnostic problems which entail transient, non-repetitive, complex (i.e., travelling wave) effects, or any combinations thereof. Additionally, accurate recovery of the surface velocity is predicated on the assumption that the measurement beam is substantially perpendicular to the surface. Where the incident beam makes an angle with the surface, the velocity estimate is in error by an amount proportional to the cosine of the angle which the beam makes with the surface normal. Another limitation of the current state-of-the-art LDV is its inability to measure the vibratory motion of structures comprising convoluted or non-planar shapes, in particular where these are sufficiently small, reflective and/or delicate structures, such as micro-electromechanical sensors (MEMS).
Recently, various designs for simultaneous multiple point LDV measurements on a structure at multiple locations have been proposed. However, such designs have the following limitations. They are not capable of vibration imaging where “imaging” is taken to imply a sufficient number of simultaneous measurement points to resolve the smallest scale spatial vibrations of interest over a two-dimensional (“2D”) area. Implicit in the term “imaging vibrometry” is the assumption that each individual sensor or pixel measures the surface motion at rates exceeding the highest temporal frequencies of interest and that the density of the sensor array likewise exceeds the maximum spatial vibration frequency of interest. Thus, such designs do not provide an imaging capability but instead employ, for example, a linear vibrometer array in conjunction with mechanical rotation or scanning to acquire 2D data. Such approaches are thus susceptible to the same limitations of single point scanning systems as detailed previously.
The aforementioned current state-of-the-art LDV includes, for example, (i) U.S. Pat. No. 8,446,575, entitled “Imaging Doppler velocimeter with downward heterodyning in the optical domain,” (ii) U.S. Pat. No. 7,193,720, entitled “Optical vibration imager,” (iii) U.S. Pat. No. 7,116,426, entitled “Multi-beam heterodyne laser Doppler vibrometer,” (iv) U.S. Pat. No. 7,961,362, entitled “Method and apparatus for phase correction in a scanned beam imager,” and (v) PCT Application Publication No. WO2002063237 A2, entitled “Interferometer.”
The current state-of-the-art LDVs attempt to address the limitations of single beam measurements by providing a limited extension of LDV to spatially distributed measurements, but such approaches are not capable of vibration imaging and, in addition, are only applicable to planar or substantially planar surfaces and structures. Thus, the current state-of-the-art LDV fails to provide simultaneous measurements of the real-time, full-field vibrometry motion of (i) structures with arbitrary geometry, and, in particular, structures with curved surfaces such as circular, cylindrical, spherical, or arbitrary curved two-dimensional or three-dimensional surfaces, and (ii) structures which exhibit rapid, abrupt or discontinuous variations of surface curvature. Additional exemplary structures include spherical, cylindrical and multi-faceted micro-scale MEMS, convoluted turbine blades, leading edges of aerospace control surfaces, and composite components employed in aerospace applications, such as thruster nozzles and nose cones. Therefore, there is a need to address the foregoing limitations.
The present invention addresses the foregoing limitations by introducing a conformal imaging vibrometer (“CIV”) that extends real-time imaging vibrometry to any structural geometry of practical interest which is not amenable to measurement by current state-of-the-art laser vibrometers employing a fixed line of sight.